The term “diode-laser bar” is a term used by practitioners of the laser art to describe a linear array of spaced-apart diode-lasers (emitters) in a bar-like, single-crystal semiconductor substrate. The diode-lasers in the bar are characterized as having a propagation-axis (emission-axis), and fast- and slow-axes perpendicular to each other and perpendicular to the propagation-axis. An emitter emits in the fast-axis with a relatively high divergence, for example between about 20° and about 30° half-angle, compared with that of emission in the slow-axis, for example, about 5° half-angle.
A diode-laser bar generally has a length of about 10 millimeters (mm), a width of about between about 1 and 1.5 mm, and a thickness of about 100 micrometers (μm). Emitters are located in a diode-laser bar with the slow-axes of the emitters nominally aligned with each other in the length direction of the diode-laser bar. A common number of emitters in a 10-mm long diode-laser bar is nineteen. The diode-laser bar is typically characterized by a “fill-factor”, which is the total of the widths of emitter apertures in bar divided by the length of the bar. In a diode-laser bar having twenty emitters of equal width, each emitter would have a width, i.e., a slow-axis dimension, of about 150 μm. The height (fast-axis dimension) of an emitting aperture is typically about 1.5 μm, i.e., 100-times less than the width.
In this background-art discussion, and in the detailed description of the present invention that follows, reference is made to a “beam-parameter product” (BPP). The BPP is a commonly used measure of quality of a laser-beam, and is the mathematical product of a beam-waist radius and the divergence half-angle of a beam into or out of the beam-waist. A typical unit of BPP for lasers beams is a millimeter-milliradian (mm·mr). A beam is deemed to have higher quality, the lower the BPP. The BPP is essentially independent of another commonly used measure of beam quality usually referred to as “brightness”, which can be roughly described as the total intensity of radiation in a beam-waist area and solid angle.
It will be evident from the typical diode-laser-emitter aperture dimensions discussed above (effectively a beam-waist that is not rotationally symmetrical) that the slow-axis BPP of the exemplified emitter is about 15-times greater (worse) than the fast-axis BPP of the exemplified emitter. This ratio is amplified for a diode-laser bar by the number of emitters in the diode-laser bar, as the BPPs in any axis are additive.
A primary significance of BPP as a quality measure is that when focusing an individual beam, or a combined beam including a plurality of individual beams, into a circular aperture having a symmetrical (in mutually perpendicular transverse axes) numerical aperture (NA), the more symmetrical the BPP of the beam, the more the radiation, brightness being equal, that can be focused into that NA.
A circular aperture is typical of the core or cladding of an optical fiber. The high degree asymmetry of the BPP of radiation from a diode-laser bar has been a particular challenge for practitioners of the laser art desirous of focusing the radiation into an optical fiber having a doped core (a gain-fiber) for optical pumping purposes, or into an optical fiber having an un-doped core, for transport to a location at which the radiation will be used.
In optical pumping of high-power fiber lasers or laser material processing, more power is required than can be provided by a single diode-laser bar. To provide such a high power in a single source, use is made of stacks of diode-laser bars. A preferred type of diode laser bar stack diode-laser bar stack is a so-called “vertical” stack in which diode-laser bars are located “one above the other”, i.e., stacked in the fast-axis direction, whatever the actual physical orientation of the stack may be. Such a stack is advantageous from the BPP symmetry point of view, inasmuch as the fast-axis BPP is increased by the fast-axis stacking, while the slow-axis BPP stays the same. Vertical stacks including up to 26 diode-laser bars are commercially available. Light-sources including such stacks typically require complicated beam-shaping and focusing including several refractive optical elements. A light-source including a plurality of vertical diode-laser bar stacks is described in U.S. Pat. No. 8,602,592, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference.
For brightness considerations, or simply to reduce the total fast-axis dimension of a vertical stack, as close as possible a fast-axis spacing of the diode-laser bars is preferred. This complicates cooling arrangements for the diode-laser bars which are mounted on very thin shelves or sub-mounts extending from a common heat-sink. Typically, high-pressure micro-channel water-cooling is required. Such micro-channel cooling requires the use of de-ionized water to avoid corrosion problems in cooling channels. Other problems with such vertical diode-laser bar stacks include little practical flexibility to change the fast-axis separation or “pitch” of the diode-laser bars; practical inability to replace defective diode-laser bars; and difficulty of test and burn-in for individual diode-laser bars.
An alternative combination arrangement for diode-laser bars is to form a planar array of diode-laser bars on a surface of a common heat-sink. This allows close thermal contact of the diode-laser bars with the heat sink which can allow the replacement of high pressure micro-channel cooling with low-pressure macro-channel cooling. This combined with electrical isolation of the diode-laser bars from the macro-channel eliminates the need for de-ionized water, allowing normal mains water (“tap water”) to be used.
Planar arrays of diode-laser bars may have the diode-laser bars arrayed along the propagation-axis direction, or along the slow-axis direction, spaced on the heat sink. Output-beams from the diode-laser bars are collimated in the fast-axis direction by a fast-axis collimating (FAC) lens, and subsequently in the slow-axis direction by slow-axis collimating (SAC) lens arrays. Various optical arrangements can be used to “stack” the beams from the diode-laser bars in the fast-axis direction to form a combined beam, which is focused by suitable focusing optics into an optical fiber.
One particular problem with planar diode-laser arrays is that the optical distance from each diode-laser bar to the focusing optics is different, typically increasing from a shortest distance to a longest distance. Because of the inherently poor beam quality from a diode-laser bar and limitations of the FAC and SAC collimating optics, among other factors, the beams cannot be perfectly collimated. This is particularly true in the slow-axis direction.
A result of this is that the slow-axis length of the diode-laser bar beams at the focusing optics would be different for each beam. Accordingly, the cross-section of the combination of the diode-laser bar beams at the focusing optics would not-have parallel sides in the fast-axis direction, which is a less than ideal condition for filling the NA of an optical fiber. This problem, of course can be mitigated by limiting the number of diode-laser bars in a planar array, but, total power limitation aside, such limitation reduces the degree to which above-discussed BPP asymmetry can be reduced by fast-axis stacking of beams from the diode-laser bars. There is a need for planar diode-laser array light-sources which allow for improvement of BPP symmetry in combined beams presented to focusing optics.